This invention relates generally to humidifier systems which produce water vapor by passing a polyphase alternating electrical current between submerged boiler electrodes, and more particularly to a controller for regulating the operation of such a humdifier in accordance with the highest of the electrode currents drawn in each phase of the alternating current.
In electronic steam humidifiers, the current drawn through each electrode of a polyphase electrode steam boiler is determined, not only by the level and conductivity of water within the boiler, but also by the cumulative time each boiler cylinder has been operated. During operation, minerals, dissolved in the boiler water, are deposited as a precipitate on various surfaces, including the boiler electrodes, within the boiler. Over time, a substantial, electrically insulating, coating can be deposited on each electrode. As the coating builds, electrode current gradually decreases.
Because steam humidifier boilers typically operate on an intermittent, "as needed," basis, the boiler electrodes are subjected to large temperature variations as the boilers are periodically switched on and off. When power is applied to a cold boiler in which a heavy precipitate coating has formed on one or more electrodes, the coating sometimes acts as an effective heat insulator and can cause the area immediately adjacent the electrode surface to become very hot. Water adjacent the electrode surface is rapidly converted into steam which then becomes trapped between the electrode and the coating. When sufficient pressure is developed, the precipitate coating is forced from the electrode and falls away in large, random, chunks. As the precipitate coating is shed in this manner, sudden, and oftentimes large, electrode current surges result as the now-clear areas of the electrode surface are once again brought into direct contact with the boiler water. Because such shedding occurs on a random basis, the timing, frequency and magnitude of such current surges is highly unpredictable.
In prior electrode boiler steam humidifier systems, total boiler electrode current was monitored by sensing the current through only one of the, frequently many, boiler electrodes. Although this provided a reasonably accurate indication of the overall average electrode current drawn by the boiler, the periodic random shedding of the precipitate coating often resulted in significant temporary current imbalances among the boiler electrodes. If a current surge were to occur in one or more of the unmonitored boiler electrodes or phases, significant circuit damage could occur without any corrective or preventive action being taken by the system. An example of such a prior system, wherein electrode current is sensed in only one phase of the power being applied to the boiler, can be found in U.S. Pat. No. 4,262,191, issued Apr. 14, 1981 to Lepper, et al.
In view of the foregoing, it is a general object of the present invention to provide a new and improved humidifier controller for controlling the operation of an electronic steam humidifier of the type having an electrode boiler operable from a source of polyphase electrical alternating current.
It is also a general object of the present invention to provide a new and improved method for operating such an electronic steam humidifier.
It is still another object of the present invention to provide a controller and method for regulating the operation of a polyphase electronic steam humidifier wherein system damage is avoided in the event excessive electrode current is drawn, regardless of where, or in which phase, the overcurrent condition occurs.